In the fields of kinematics and mechanics, the solution to the problem of generating or sensing motion in a two-degree of freedom system with a mechanism, has been approached in a number of ways. One solution of the prior art is provided by a gimbal. An alternative system provided by the prior art is a pivotable member restrained by a system of orthogonal semi-circular sliding braces.
FIG. 1 shows a gimbal 10 of the prior art in a two-axis drive system. The gimbal 10 includes a base 12 which may be fixed to ground 11. The base 12 includes a swivel 17 with a bracket 14 attached thereto. The bracket 14, accomplishes rotary motion about the swivel 17 relative to the base 12. Attached to the bracket 14 is a second swivel 19 with an output member 22 attached to a shaft 20 in axial alignment with the swivel 19. In the gimbal 10 of the prior art, a first motor 16 is located on the base 12 to generate angular motion of the bracket 14 with respect to the base 12. A second motor 18 is positioned to provide motion about an axis generally orthogonal to the first motor 16. As the motors 18 and 16 are driven, the output member 22 responds accordingly.
The principle deficiency of the gimbal 10 stems from the need to locate the second motor 18 on the rotating bracket 14. Thus, for the prior art gimbal 10 to generate motion for the output member 22, not only is a motor 16 required on the base 12 but also a motor 18 must be mounted on the bracket 14.
To communicate data from the second motor 18 to a controller attached to ground 11 may require difficult and complex connections. Such connections may interfere with the operation of the gimbal 10. Difficult connections may also contribute to reduced reliability of the gimbal based, two-axis drive mechanism. The weight of the gimbal 10 is also increased by the weight of motor 18 making it less adaptable to various kinematic drive tasks.
FIG. 2 shows a method of sensing two-axis motion of a pivoting member 24. The mechanism of FIG. 2 provides a central pivoting member 24 which is restrained by a first semi-circular sliding brace 30 and a second semi-circular sliding brace 32. First semi-circular sliding brace 30 is attached to ground 11 by pivots 38 and 39 which allows the brace 30 to rotate about a first axis 40. A second semi-circular sliding brace 32 is attached to ground 11 by pivots 42 and 44 which allow the brace 32 to rotate about a second axis 46. The second axis 46 is orthogonal to the first axis 40 to provide sensor outputs in two axes. The member 24 is located within the intersection of the two semi-circular sliding braces 30 and 32. The sliding braces 30 and 32 work in a radial fashion such that the member 24 is free to move within the sliding braces 30 and 32 while being constrained by the sliding braces 30 and 32 and bearing 36. The axial sensors 41 and 43 for this configuration are attached to shafts 28 and 26 along axis 46 and 40, respectively. The shafts 28 and 26 are then connected to sliding braces 32 and 30, respectively. The pivoting member 24 is attached to a base 34 at pivot 36. All pivoting member 24 motion relative to the base 34 is sensed by sensors 41 and 43 as shafts 28 and 26 turn, respectively.
The principle deficiency of the sliding brace system 15 stems from the tendency of the sliding braces 30 and 32 to bind and/or deform and cause backlash thus reducing the accuracy and repeatability of the device. The sliding brace method does provide a way to hold the two-axial sensors 41 and 43 stationary which creates an advantage over the gimbal 10 previously described.
Improvement over the sliding brace method are described in U.S. Pat. No. 4,723,460 to Rosheim and in U.S. Pat. No. 2,929,258 to Mackway. In both cases, sliding braces have been replaced with an inner and outer rotary bearing ring. The outer race of the inner bearing ring is coupled to the inner race of the outer bearing ring, thereby allowing rotation between the two bearing rings. However, the mechanisms described in Rosheim and Mackway rely on press fits for assembly. This makes them less suitable for transmitting large forces and resisting abuse. Rosheim and Mackway also both use a single rotary bearing for transmitting rotation between a pivoting member and shafts for imparting motion thereto or sensing motion thereof. The inner race of the single rotary bearing is twisted out of the plane of the outer race, increasing wear and subsequently producing backlash; the backlash increasing as the bearings wear.
In summary, the prior art suffers from a number of drawbacks. The gimbal system of FIG. 1 suffers in that the second motor 18 needs to be incorporated in the moving bracket 14. This adds substantially to the weight of the mechanism and thus the cost and difficulty of manufacture. The mechanism of FIG. 2 includes the problems of difficulty of mechanism repeatability, difficulty of construction because the rails need to be fairly small in relation to the pivoting member 24 to accomplish quick motion and the mechanism has a degree of backlash because the brackets 30 and 32 tend to bind and/or deform. In addition, the mechanisms of Rosheim and Mackway make use of press fits which simplify assembly, but which are a failure point along with the single rotary bearings used in the bearings rings which are subject to ever increasing backlash, especially under large loads.
Thus, it is a motive of the present invention to provide a two-axis motion mechanism that provides a robust, backlash reduced means of generating or sensing two-axis motion while keeping the position of either the drive motors or sensors stationary, depending upon whether the mechanism is used as a drive or sensor mechanism.